1. Field of the Invention
The present invention relates to a polishing technique for flattening the surface of a semiconductor device, and more particularly to surface treatment or conditioning of a polishing cloth used for flattening the surface of a semiconductor device.
2. Description of Related Art
In order to form a semiconductor integrated circuit device having a multilayer structure in which wiring layers are arranged in a three dimensional manner, it is necessary to flatten the surface of an interlayer insulating film such as a silicon oxide film between the wiring layers. More specifically, if a silicon oxide film is deposited on a first alminium wiring layer as a bottom layer by a CVD method, an irregular portion would appear on the surface of the silicon oxide film due to the first wiring layer. In this state, if a second aluminium wiring layer is formed on the surface of the silicon oxide film by use of a photolithography process and a dry etching process, a focus cannot be maintained on the surface in the vicinity of the irregular portion in the patterning of a resist layer, or a step portion between the irregular portion and the other portions remains after the dry etching process. For this reason, the irregular portion on the surface of an interlayer insulating film should be flattened, as disclosed in the Japanese Laid-Open Patent Disclosure (JP-A-Hei5-315308). That is, a type of process solution containing a polishing agent is dropped on a polishing cloth provided on a rotating surface table and the irregular portion on the surface of the interlayer insulating film is pushed against the polishing cloth and thereby the irregular portion is removed. The polishing of the silicon oxide film proceeds based on a chemical etching action to the silicon oxide film and a mechanical action such as abrasion of the silicon oxide film by particles of the polishing agent. For this purpose, a process solution is used in which silica particles having a particle size as great as 20 nm are dispersed into an ammonia solution as the polishing agent, wherein the particles comprise about 10 to 30 wt %, as disclosed in the Japanese Laid-Open Patent Disclosure (JP-A-Hei4-75338).
The polishing cloth needs to have hard and flat surface properties in order for it to be used to remove a small surface irregular portion. As such a polishing cloth, there is used a fiber type of polishing cloth such as a polyurethane-impregnated polyester fiber cloth in which synthetic fiber is hardened, or a foam type of polishing cloth such as urethane foam in which polymer is foamed.
In the fiber type of polishing cloth, there is a small space between fibers and the small space extends in a channel manner throughout the whole polishing cloth. Thus, the silica particles contained in the process solution and shavings from the interlayer insulating film are exhausted from the peripheral portion of the polishing cloth through the space channels. Therefore, the polishing cloth is less likely to be binded on the surface. However, the fiber type of polishing cloth has a reduced hardness because of the space channel and is not very adaptive.
On the other hand, in the foam type of polishing cloth, if the density of spherical bores formed through foaming is varied, the hardness of the polishing cloth can be varied, thus resulting in an adaptive polishing cloth. However, polishing particles (silica particles) and shavings often remain in the bores existing in the vicinity of the surface of polishing cloth when a plurality of semiconductor substrates are continuously polished. Therefore, there is the drawback that scratches are caused on the surface of an interlayer insulating film, or the polishing process speed for the insulating film is decreased.
In order to avoid the binding of the foam type of polishing cloth, a polishing cloth surface conditioning process needs to be performed, i.e., a process of cutting an extremely thin surface layer of the polishing cloth for every polishing process needs to be performed to expose the non-binded surface of the polishing cloth. In a conventional conditioning or surface-treating process, a diamond-electrodeposited layer 110, in which diamond particles 16 having the grain size in the range 1 to 100 .mu.m is electrodeposited on a circular stainless plate 120, is rotated as shown in FIG. 1A and pushed against the foam type of polishing cloth which is rotated while pure water or process solution is flowed thereon. When a tool using the electrodeposited diamond particles is pushed against the rotating polishing cloth, as in the conventional polishing cloth surface treatment process, there is a problem in that some of the diamond particles drop and become buried in the polishing cloth so that scratches are caused on the interlayer insulating film surface because of the dropped diamond particles. Also, there is another problem in that the polishing cloth is polluted with metal because the diamond particles are electrodeposited on the metal plate such as stainless plate. Ni is used for electrodeposition of the diamond particles to the stainless plate and, if a diamond particle drops, Ni dissolves into the process solution, resulting in pollution of a semiconductor substrate. The semiconductor substrate is also polluted because of dissolving or peeling of metal such as Ni from the side wall of the stainless plate because the diamond particles are electrodeposited on the undersuface of the stainless plate.